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The optical pumping technique manipulates
the populations in the hyperfine levels of the ground state by exciting
transitions to higher principal quantum states with infrared, or higher
frequency, light. As shown above, the
atoms in one hyperfine level are excited optically to a higher state from
which they decay spontaneously to both ground state hyperfine levels. The population of the hyperfine state
involved in the stimulated transition is rapidly depleted; the population of
the second hyperfine level is enhanced.
Microwaves applied at the 9.192…GHz frequency can be locked to the
atomic transition frequency, as in a Cs beam standard.
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Optical pumping has both advantages and
disadvantages compared to magnetic state selection. On the positive side, it can be
accomplished in a more compact device and it can enhance the number of atoms
in the desired state rather than just rejecting the atoms in the undesired
state. On the negative side are
increases in complexity, the difficulty of obtaining laser diodes at the
proper frequency and with suitable stability (as of 1999), and some
additional performance-degrading mechanisms.
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Optical pumping can eliminate the need for
state-selection magnets, and result in a larger number of atoms contributing
to the signal which results in a superior signal-to-noise ratio. In addition, the spatial symmetry of the
optical pumping reduces certain frequency shifts.
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L. L. Lewis,
“Miniature Optically Pumped Cesium Standards,” Proc. 45th Ann. Symp. on
Frequency Control, pp. 521-533, 1991.
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P. J. Chantry, B. R.
McAvoy, J. M. Zomp, and I. Liberman, “Towards A Miniature Laser-pumped Cesium
Cell Frequency Standard,” Proc. 1992 IEEE Int’l Frequency Control Symp., pp.
114-122, 1992.
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P. J. Chantry, I.
Liberman, et. al, “Miniature Laser Pumped Cesium Cell Atomic Clock
Oscillator,” Proc. 1996 IEEE Int’l Frequency Control Symp., pp. 1002-1010,
1996.
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